1. Field of the Invention
This invention relates to a developing roller for use in electrophotographic and electrostatic recording apparatus such as copying machines and printers. It also relates to a method and apparatus for developing electrostatic latent images using the developing roller.
2. Prior Art
With the recent advance of electrophotography, severer requirements are imposed on conductive members used in various steps of the electrophotographic process. The developing roller used in the developing device is one of such important conductive members. The developing roller is required to have a desired electrical resistance and adequate characteristics for various developing mechanisms.
In the prior art electrophotographic process, a method is employed for developing electrostatic latent images using a non-magnetic one-component developer as a developer or toner. One typical developing method is an impression developing method wherein a donor roller receiving a toner is placed in pressure contact with a photoconductor drum having a latent image borne thereon whereby the toner is delivered from the donor roller to the drum, adhering the toner to the latent image on the drum to form a visible toner image. This method allows for simplification and size reduction of apparatus and the use of color toner because no magnetic material is needed.
The impression developing method requires the developing roller to be formed of a conductive elastomer since development is carried out by placing the developing roller having a toner carried thereon in pressure contact with the photoconductor drum having a latent image borne thereon, thereby causing the toner to adhere to the latent image on the drum.
Referring to FIG. 1, the impression developing method is illustrated. A developing roller 1 is placed between a toner feed roller 4 for feeding a toner 6 and a photoconductor drum 5 having an electrostatic latent image borne thereon and in contact with the photoconductor drum 5. Upon rotation of the developing roller 1, photoconductor drum 5, and toner feed roller 4, the toner 6 is fed from the feed roller 4 onto the surface of the developing roller 1 and regulated into a uniform thin layer by a doctor blade 7. The thin layer of toner is then delivered from the developing roller 1 to the photoconductor drum 5 to adhere to the latent image whereby the latent image is developed into a visible toner image. The toner image is finally transferred from the photoconductor drum 5 to a record medium, typically paper in a transfer section 8. Also included is a cleaning section 9 having a cleaning blade 10 for scraping off the toner left on the photoconductor drum 5 after the transfer step.
During rotation, the developing roller 1 must maintain close contact with the photoconductor drum 5. Then the conventional developing roller 1 is of a structure having a conductive layer 3 around a shaft 2 as shown in FIG. 2. The shaft 2 is of a highly conductive material, typically metal. The conductive layer 3 is formed of a conductive elastomer in the form of an elastic rubber such as silicone rubber, acrylonitrile-butadiene rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM) or a sponge such as urethane foam, with a suitable conductive agent being blended therein. The developing roller 1 is prepared by applying the conductive elastomer onto the outer periphery of the shaft 2 to form the conductive layer 3 and grinding the surface of the conductive layer 3 with an abrasive wheel 11 as shown in FIG. 3.
However, the prior art development methods had the following problems resulting from the properties of developing rollers used therein. (1) The conductive layer 3 of the developing roller 1 which is formed of an elastic rubber such as silicone rubber, NBR and EPDM is readily abradable and thus susceptible to slight unevenness of grinding during the roller manufacturing process, which can result in positional variations in the amount of toner carried on the roller. (2) On long-term operation, as the developing roller on the surface is abraded away due to friction with the regulating blade and photoconductor drum, the developing roller surface is smoothened to reduce the amount of toner carried on the roller. (3) When a developing roller having a conductive elastomer layer of elastic rubber such as silicone rubber, NBR, EPDM and polyurethane resin around a conductive shaft is used without a surface or coating layer on the conductive elastomer layer, the developing roller has a higher coefficient of friction with the photoconductor drum or regulating blade, which requires a greater power for driving and prevents the relative speed of the developing roller and photoconductor drum from being stabilized, resulting in image variations. (4) One countermeasure to (3) is to form a surface or coating layer on the conductive elastomer layer for providing a reduced coefficient of friction, although no sufficient triboelectric charging of toner then takes place between the developing roller and the regulating blade, leading to the risk of image fogging. For the reasons (1) to (4), there arise problems such as density variations of a resultant image and character thinning after long-term operation. This tendency is outstanding particularly with the impression development method using a non-magnetic one-component toner.